It is well known that defects in parts of ferromagnetic material may be detected by magnetizing the part to cause localized magnetic leakage fields to occur at points on the surface of the part where there are cracks or the like forming magnetic discontinuities. Such leakage fields are detected by dispersing finely divided magnetic particles over the surface of the part, the particles being concentrated at regions where the leakage fields are produced. The leakage fields may also be detected through the use of a suitable leakage field sensing probe, in certain applications.
The results of such testing are used in determining whether a given part will be satisfactory for its intended use and where the part is to be used in a critical application and defects might lead to failure thereof, reliable testing may be essential for safety purposes.
To insure that favorable results from a given test provide an accurate indication that the part is actually free of defects, it is essential that the level of magnetization within a part be of ample magnitude, and various rules and techniques have been used or proposed for this purpose. For example, when a part is to be magnetized by the direct passage of current through the part or through a conductor extending centrally through a part, a rule of thumb is applied that the current should be 1000 amperes per inch of part diameter. This rule is generally satisfactory for small to medium size parts of regular shapes but with large diameter parts, it can result in unnecessarily high amperages and in some cases amperages that are greater than obtainable with existing power pack capabilities.
For coil magnetization, a rule may be applied that the ampere-turn requirements are calculated by dividing 45,000 by the ratio of the length of the part to the diameter of the part. This rule may be applied relatively easily and with good results to simple cylindrically shaped parts but when the parts have complex shapes, determining what actually constitutes the length to diameter ratio can cause confusion.
Instruments have also been used including fluxmeters, Hall effect instruments, artificial crack indicators, and eddy current instruments. The fluxmeter requires a search coil wound around an area where a measurement is to be made and is dependent upon the equal distribution of magnetic flux across the area enclosed by the search coil. Also it requires consideration of the relationship between the measured flux density and the optimum flux density for the particular material being tested.
Hall effect instruments cannot measure the flux density of fields contained within a part and are limited to arrangements in which an opening can be introduced into a part for probe insertion or in which the tangential magnetic field intensity is measured at the surface, the magnetization current being determinable from such a measurement if the magnetic characteristics of the material are known.
Various types of devices have been provided for artificially creating magnetic discontinuities, such devices being placed on the surface of a part that is being subjected to a magnetizing force with the build-up of an indication on the device being used to indicate an adequate magnetizing current. Such devices are of limited reliability, and require a considerable amount of judgment on the part of an operator.
Eddy current instruments have also been proposed for the purpose of measuring magnetic flux density in parts, one disclosure being provided in Technical Report AFML-TR-72-115 of the Air Force Materials Laboratory, Air Force Systems Command, Wright-Patterson Air Force Base, Ohio, May 1972. That report discusses experimental results indicating that a continuous, single-valued, eddy current signal change is generated for changing flux densities in a ferromagnetic steel sample and indicates that heat to heat variations in hardness, stress and cold work do not significantly affect flux measurement by eddy currents. However, it is indicated that gross hardness changes such as encountered in different heat-treated materials will affect the readings and also that the type of steel will affect the readings so that a calibration curve for each type of steel and heat-treat would be required to relate changes in flux density to the eddy current responses. In addition, it would be necessary to control other factors including surface conditions such as scale or work hardening and the magnetic state of the part, as discussed briefly in Schroeder "Magnetic Flux Density Measurements Relative to Magnetic Particle Testing," report on Symposium at the National Bureau of Standards, May, 1976 published by the American Society for Testing Materials.
It is also known that Barkhausen noise impulses are attributed to abrupt changes in the magnetization of a substance when the magnetizing field is gradually altered on the steep part of the magnetization or hysteresis loop, but so far as is known, no application of the effect to magnetic flux density measurements has been proposed.
Still other approaches have been used for attempting to obtain an optimum level of magnetizing current. Operators with a great deal of experience can in many cases make an accurate estimate of the proper amount of magnetizing current when parts to be tested are not highly irregular in shape. It is also possible in some cases to increase the magnetizing current until background indications are obtained and to then "back-off" the current to a certain extent. In addition, it is always possible to try to form a crack in a part of a minimum acceptable size, or to try to find a part having such a defect, and then establish the magnetization level. Such approaches have limitations. Highly experienced operators are not always available and even operators with a great deal of experience can make errors in judgment especially with regard to parts which are highly irregular in shape. Attempting to form a defect for measurement or to find a part having a defect suitable for measurement can be time-consuming and is a procedure which is difficult if not impossible to pursue in many cases.
The result is that practically all parts are tested through the established rules of thumb and there has been no sure and satisfactory way of establishing the proper magnetizing current, especially in the case of parts of irregular shape.